NEUTRON PEAK IN THE EXTENDED-SADDLE-POINT MODEL OF HIGH-TEMPERATURE SUPERCONDUCTORS

An explanation is proposed of the maximum at 41 meV in the inelastic spin-flip neutron scattering from YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}}, based on the extended-saddle-point model developed by the author in his previous works. It is shown that for appearance of the maximum in the imaginary part of the spin susceptibility a close proximity of the Fermi energy to the extended saddle point is necessary. The energy of the maximum is then close to 2{Delta}{sub max} in agreement with experiment. Theoretical and experimental evidence concerning the energy of the extended saddle point (flat region) is discussed. Different limiting cases are calculated. A proof is given within the present model that interaction in the final state is small, and hence, no collective modes are formed. A general discussion of the experimental situation is presented. {copyright} {ital 1998} {ital The American Physical Society}